In many a.c. powered devices, it is desirable to bidirectionally control the rotation of a d.c. motor. One such example is a photgraphic slide projector, wherein a d.c. motor is used to focus a projection lens. In such a projector, the d.c. motor must be capable of rotation in either direction to properly focus the projection lens.
FIG. 1 shows a known circuit 10 for using an a.c. power supply 12 to bidirectionally control the rotation of a d.c. motor 14. Motor 14 comprises, for example, a conventional, permanent magnet type d.c. motor capable of rotation in one of two directions depending on the polarity of current applied thereto. Circuity 10 is utilized, for example, in a photographic slide projector (not shown) for focusing a projection lens (not shown) responsive to the operation of an infrared transmitter 16 by a human viewer (also not shown). Coded infrared information 18, output by transmitter 16, is received and decoded by an infrared receiver 20.
Infrared transmitter 16 and receiver 20 comprise standard components, the receiver including, for example, an IR sensor 22, preamp 24, decoder 26, and interface logic 28 connected generally seriatim. Logic circuit 28 typically comprises CMOS logic, wherein a logical low voltage is typically in the range of about 0 volts, and a logical high voltage is typically in the range of about 5-15 volts. Logic 28 could comprise, however, any type of two state, positive logic. The output of receiver 20 is used to control a motor controller circuit 30, so as to control the direction and amount of rotation of motor 14.
Continuing to describe FIG. 1, power supply 12 includes a source of a.c. power 32, for example 120 volt line voltage of the type typically used in the United States. A.c. source 32 is connected across a primary 34A of a transformer 34. A secondary 34B of transformer 34 is connected between a first terminal 36 of motor 14 and a circuit common junction 38.
Circuit 30 includes a first optically coupled, silicon controlled rectifier (SCR) circuit 40 including an SCR 41 having its cathode connected to a second terminal 42 of motor 14, and its anode connected to common junction 38. A light emitting diode (LED) 43 is positioed so as to be optically coupled to SCR 41 for selectively enabling the SCR, and is connected between an output A of receiver 20 and common junction 38. A resistor R.sub.1 is connected in series with the anode of LED 43 for limiting the current flow therethrough, and a resistor R.sub.2 is connected between the cathode and gate of SCR 41 for controlling the sensitivity of the gate.
A second optically coupled, silicon controlled rectifier circuit 44 includes a second SCR 45 connected in the reverse polarity between motor terminal 42 and common junction 38. A second LED 46 is positioned to be optically coupled with SCR 44, and is connected between an output B of receiver 20 and common junction 38. A resistor R.sub.3 is connected in series with the anode of LED 46 for limiting current flow therethrough, and a resistor R.sub.4 is connected between the gate and cathode of SCR 45 for adjusting the sensitivity to triggering of the SCR.
In operation, responsive to selectable codes emitted by transmitter 16, output A or B of receiver 20 is selectively driven to a high logic level, enabling the rotation of motor 14 in one of two directions. Examining this operation in greater detail, if output A is driven to a logic high, a current path is developed through resistor R.sub.1 and LED 43 to common junction 38, activating the LED and enabling rectifier 41. Negative a.c. pulses appearing at secondary 34B of transformer 34 are then passed through rectifier 41, and operate to power motor 14 for rotation in a first direction. Since rectifier 45 is not enabled, positive a.c. pulses are blocked.
With output A held to a low logic level, and output B driven to a high logic level, current flows through resistor R.sub.3 and LED 46, lighting the LED and enabling rectifier 45. In this mode of operation, positive a.c. pulses developed at transformer secondary 34B are passed by rectifier 45, with the negative pulses being blocked by rectifier 41. Under these circumstances, motor 14 is powered to rotate in the opposite direction. Motor 14 is thus bidirectionally controlled using the power supplied by a.c. power supply 12. It is, of course, undesirable to drive both outputs A and B simultaneously to a high logic level, and the operation of transmitter 18 and receiver 20 is designed to prevent such an occurence.
The above described circuit, while operating adequately to drive motor 14, presents the disadvantage of relatively expensive. More particularly, optocoupled rectifiers 40 and 44 are expensive devices, particularly when used in a mass production device.